Scabies - diagnosis and management

Scabies manifests as an itchy skin rash, as a result of an allergic reaction to the female scabies mite. Infection is easily spread from person to person with direct skin contact. Clinical diagnosis can be made if burrows are observed on the skin. Permethrin and malathion are effective treatments for the rash

Scabies transmission occurs when there is transfer of a fertilised female mite by direct, (approximately five minutes)
skin-to-skin contact with an infected person.

Diagnosis is usually made clinically. Laboratory diagnosis is not usually necessary but may be useful for uncertain
cases or cases in residential care.

Malathion and permethrin are effective treatments for scabies.

All recent contacts should be treated.

The itch may persist for weeks even though the mite is gone. However itch beyond six weeks may indicate treatment
failure.

Scabies is caused by the female scabies mite (Sarcoptes scabiei). The itchy skin rash is due to an allergic
reaction that occurs to the mite’s trail of debris, faeces and saliva. Scabies mites occur worldwide and are prevalent
in New Zealand. Scabies infestation can affect all socioeconomic groups and is not a result of poor hygiene. It is however,
more often associated with poverty and overcrowding.1

Transmission of scabies usually occurs by the transfer of fertilised female
mites

Scabies transmission occurs when there is transfer of a fertilised female mite by direct, prolonged (approximately five
minutes) skin-to-skin contact with an infested person. Infection is easily spread to sexual partners and household members.

Transfer can also result from sharing clothing, towels and bedding as the mite can live for up to two to three days
away from the human body.

In children, scabies transmission most commonly occurs at day-care centres, schools or sleepovers while in elderly people
it most commonly occurs in residential care.

Life cycle of scabies mite

After mating the male mite dies. Newly mated female mites will lay two to three eggs per day (for her lifespan of one
to two months) in burrows within the stratum corneum. After approximately two to four days, the eggs hatch and the larvae
leave the burrow to stay on the surface of the skin, or in short burrows until they reach maturity in seven to nine days.

Diagnosis is usually made clinically

Scabies infestations can be difficult to diagnose. It should be considered whenever a patient complains of severe itch
on the trunk and limbs, particularly when the visible signs are minor. Exposure to an infested person should promote a
high index of suspicion.

There is usually a history of intense itch, worse at night and after a hot shower/bath.

The itch related to scabies can start at variable times after a person becomes infested, from hours (if the person has
been infested before and therefore previously sensitised) to several weeks in an initial infestation.

Fig 1: Burrows on foot of young adult patient

Fig 2: Scabies burrows with arrows to show where the mite can be seen on magnification

Fig 3: Dermoscopy reveals tiny grey triangles (the head of the female mite) at distal ends of burrows
on fingers

A confident clinical diagnosis can be made if burrows are observed on the wrists, finger web spaces and/or on the sides
and soles of the feet (Figure 1). Irregular clusters of inflammatory nodules in the axillae, genitalia or thighs are also
highly suggestive of infestation.

Burrows are 5–10 mm long and they look like greyish pencil marks (Figure 2) on pale skin (in darker skin they may appear
pale). Burrows can be difficult to identify when the skin has been scratched, is secondarily infected or in the presence
of eczema.2,3 Burrows are best seen under magnification. Dermoscopy may reveal tiny grey triangular structures
at the leading edge of the burrow (Figure 3).4

The rash is often widespread and polymorphic; there may be scratched papules and nodules, eczema, folliculitis and urticaria,
usually sparing the head and neck. In infants, elderly and immunocompromised people, scabies may also affect the face
and scalp.2 Vesicles and pustules on the palms and soles are characteristic of scabies in infants, and may
persist for several weeks after the mites have been successfully destroyed.5

Apply ink to burrows2

Rub a non-toxic water-soluble felt pen over an area suspected of having burrows, wait a few moments and then wash
off ink.

In the presence of a burrow the ink will track down the burrow, forming a characteristic dark, zig-zag line.

Crusted scabies may occur in elderly, immuno-compromised or institutionalised people

Elderly, immunocompromised or institutionalised people may present with crusted or “Norwegian” scabies, a variant of
scabies where extensive hyperkeratosis occurs (Figures 4 and 5). The diagnosis is often delayed because itch may be less
severe and typical papules and nodules are frequently absent. The rash may resemble psoriasis. Thousands or even millions
of mites are present in the crusts making this type of scabies easily transmissible. Crusted scabies is a common cause
of institutional outbreaks (e.g. rest homes, prisons, or hospitals). Staff who are even minimally exposed to someone with
crusted scabies (e.g. laundry workers, cleaning staff) are at risk of infestation.6

Laboratory diagnosis is not usually necessary

Microscopy of burrow contents, or scrapings from the hands of a patient with crusted scabies, may reveal mites, eggs
or faeces. Laboratory diagnosis may be useful for scabies in residential care or in cases where the diagnosis is uncertain.
However, even experienced dermatologists only recover a mite or egg in about 50% of scabies cases.2,4

Treatment – malathion and permethrin are effective treatments for scabies

Permethrin and malathion are the most frequently used treatments for scabies. While both have been used extensively,
the best evidence is for permethrin.7 Researchers were unable to draw conclusions about malathion’s effectiveness
as there were no trials involving malathion.7

Gamma benzene hexachloride (Lindane) has been associated with aplastic anaemia and convulsions, possibly due to its
application to broken skin. Lindane has been withdrawn in the UK and in Australia.7

Fully funded scabicides available in New Zealand are:

5% Permethrin cream – Left on for 8–14 hours before washing off. Reapplied after seven days.8 Permethrin
is a safer choice in pregnancy, lactating women and infants because of its low inherent toxicity and low percutaneous
absorption.9

0.5% Malathion lotion – Left on for 24 hours before washing off. Reapplied after seven days.8

1% gamma benzene hexachloride cream (Lindane) – Left on for 8–12 hours. Not reapplied. Lindane should only be used
if other treatments have failed, and should not be used in patients weighing less than 50 kg, those with a seizure disorder
or pregnant and lactating women.

Scabicides should be applied to the entire body from the chin and ears downwards. The face and scalp should also be
included for infants under two years, people who are immunocompromised and elderly people (but avoiding contact with eyes).
Particular attention should be paid to the area between toes and fingers, genitals and under nails (a soft nail brush
may be necessary).8 This rarely causes stinging or irritation. Treatment needs to be reapplied to areas that
are washed within the necessary application time (such as after hand washing). It can be helpful for a second person to
assist with the application to areas that are not easily accessible.

Note: The BNF recommends application of scabicides to the entire body, including the head and neck for all people.8

Immunocompromised patients and those with crusted scabies may prove resistant to repeated topical therapy and require
systemic insecticide therapy such as oral ivermectin (200 mcg/kg).1,9

Retreatment may be necessary if symptoms and signs persist; or after oral antibiotics if there is crusting due to secondary
impetigo.

Reducing transmission – treat all recent contacts

Household members and anyone with recent direct and prolonged body contact should be treated at the same time even if
they are not itchy. This is because infestation may occur up to several weeks before symptoms and secondary rash appear.

Clothing, sheets, pillow cases, towels and facecloths that have been in contact with the patient within the previous
few days should be machine washed in hot water and dried (hot cycle) or dry cleaned.

It is not generally necessary to wash blankets, duvets or quilts. They can be hung outside in the sun for a day. There
is no need to treat furniture or carpets with an insecticide, except in the case of crusted scabies where numerous mites
may be found on fomites.

The itch may persist for weeks even though the mite is gone

Do not assess treatment response until four weeks after treatment is finished. Overtreatment with scabicides can cause
skin irritation and contact eczema.6

Itch or rash may persist for weeks after treatment due to the continuing allergic reaction to persisting antigens within
the skin. Oral antihistamines, crotamiton (Eurax) cream, emollients and mild to moderate potency topical steroids can
be useful.

Fig 6: Nodular scabies

Itch beyond six weeks after initial treatment may indicate treatment failure (particularly if itch persists at the same
level or is increasing in intensity). This could be due to re-infestation, inadequate treatment of contacts, resistance
to therapy, or an incorrect initial diagnosis. Consider an alternative diagnosis and re-examine the person. If the diagnosis
of scabies is established, a different scabicide should be tried if all contacts were originally treated simultaneously,
and the treatment was correctly applied.2

Even after successful treatment, pruritic nodules may persist in some people. Nodules are usually brownish red, can
be up to 2 cm in diameter and are most often seen around the genitals and axillae (Figure 6). Treatment with topical corticosteroids
may be useful.9

People with scabies can become secondarily infected with streptococci or staphylococci, which should be treated with
oral antibiotics for seven days. Flucloxacillin is recommended as empirical treatment, erythromycin is an alternative
for those with penicillin allergy.2

Images contributed by NZ DermNet, the website of the New
Zealand Dermatological Society.